How do I convince my girl to start riding?

+1, if she can muster some enthusiasm for cycling then visit some bike shops together and let her tell you what kind of bike and riding (parks, pubs, commuting, training, track) she's interested in. Trying to manipulate her into being an unwilling fixie skidder is going to end in tears

Plus one too. Being pushed into something you're not really interested in is not nice.